This invention relates to flowmeters, and more particularly to those utilizing vortex shedding.
Vortex shedding is a phenomenon which occurs when a fluid flows past a bluff (unstreamlined) body. Boundary layers of a slow-moving viscous fluid are formed along the outer surface of the body, and because it is not streamlined, the flow cannot follow the contour of the body but separate themselves from the body, become detached, and roll themselves up into vortices. A vortex of one rotational sense is shed from one side of the body. The next vortex to be shed is then shed with an opposite rotational sense. The rate of vortex shedding is proportional to flow rate. While the phenomenon has long been known, an impediment to the utilization of the phenomenon for measuring flow rates has been the difficulty in detecting the vortices.
One form of prior art flowmeter of the above-described type employs a special construction of bluff body to detect the vortices as they are formed. For this purpose the body is provided with flexible side walls, each forming one plate of a capacitor. The other such plate is located inside the bluff body. The formation of a vortex produces an area of reduced pressure on one side of the body, and a corresponding area of increased pressure on the other. This pressure differential and its reversal, when the next succeeding vortex begins to form, is detected by the capacitance change resulting from the flexure of the side walls.
According to the present invention, there is provided a flowmeter utilizing vortex shedding and having a light source coupled with a detector via a length of optical fiber including an unclad portion positioned in the path of vortices shed from a bluff body and wherein the output of a detector is fed to the input of a pulse repetition frequency meter. The light source is not necessarily an emitter in the visible region of the spectrum but may alternatively be an IR or UV emitter.
In the present invention the bluff body merely serves to generate the vortices, and the vorices themselves are detected, rather than their generation.
When an unclad optical fiber is immersed in a medium of lower refractive index, that medium acts as a cladding for the fiber in providing a guiding interface for light propagating aong the fiber. When, however, the fiber is immersed in a medium whose refractive index is equal to or greater than that of the fiber, the interface is no longer guiding, with the result that the optical attenuation of the fiber is increased. A vortex produces a localized change in refractive index in a medium and hence when a vortex comes into contact with an unclad fiber having a refractive index not much greater than that of the bulk medium, the optical attenuation of that fiber is temporarily modified until the vortex dies out or moves away.
The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.